Phylogeography, more than elevation, accounts for sex chromosome differentiation in Swiss populations of the common frog (Rana temporaria).

Sex chromosomes in vertebrates range from highly heteromorphic (as in most birds and mammals) to strictly homomorphic (as in many fishes, amphibians, and nonavian reptiles). Reasons for these contrasted evolutionary trajectories remain unclear, but species such as common frogs with polymorphism in the extent of sex chromosome differentiation may potentially deliver important clues. By investigating 92 common frog populations from a wide range of elevations throughout Switzerland, we show that sex chromosome differentiation strongly correlates with alleles at the candidate sex-determining gene Dmrt1. Y-specific Dmrt1 haplotypes cluster into two main haplogroups, YA and YB , with a phylogeographic signal that parallels mtDNA haplotypes: YA populations, with mostly well-differentiated sex chromosomes, occur primarily south of the main alpine ridge that bisects Switzerland, whereas YB populations, with mostly undifferentiated (proto-)sex chromosomes, occur north of this ridge. Elevation has only a marginal effect, opposing previous suggestions of a major role for climate on sex chromosome differentiation. The Y-haplotype effect might result from differences in the penetrance of alleles at the sex-determining locus (such that sex reversal and ensuing X-Y recombination are more frequent in YB populations), and/or fixation of an inversion on YA (as supported by the empirical observation that YA haplotypes might not recombine in XYA females).

An organelle gatekeeper function for Caenorhabditis elegans UNC-16 (JIP3) at the axon initial segment.

Neurons must cope with extreme membrane trafficking demands to produce axons with organelle compositions that differ dramatically from those of the cell soma and dendrites; however, the mechanism by which they accomplish this is not understood. Here we use electron microscopy and quantitative imaging of tagged organelles to show that Caenorhabditis elegans axons lacking UNC-16 (JIP3/Sunday Driver) accumulate Golgi, endosomes, and lysosomes at levels up to 10-fold higher than wild type, while ER membranes are largely unaffected. Time lapse microscopy of tagged lysosomes in living animals and an analysis of lysosome distributions in various regions of unc-16 mutant axons revealed that UNC-16 inhibits organelles from escaping the axon initial segment (AIS) and moving to the distal synaptic part of the axon. Immunostaining of native UNC-16 in C. elegans neurons revealed a localized concentration of UNC-16 at the initial segment, although UNC-16 is also sparsely distributed in distal regions of axons, including the synaptic region. Organelles that escape the AIS in unc-16 mutants show bidirectional active transport within the axon commissure that occasionally deposits them in the synaptic region, where their mobility decreases and they accumulate. These results argue against the long-standing, untested hypothesis that JIP3/Sunday Driver promotes anterograde organelle transport in axons and instead suggest an organelle gatekeeper model in which UNC-16 (JIP3/Sunday Driver) selectively inhibits the escape of Golgi and endosomal organelles from the AIS. This is the first evidence for an organelle gatekeeper function at the AIS, which could provide a regulatory node for controlling axon organelle composition.

Does the speciation clock tick more slowly in the absence of heteromorphic sex chromosomes?

Squamates may be an attractive group in which to study the influence of sex chromosomes on speciation rates because of the repeated evolution of heterogamety (both XY and ZW), as well as an apparently large number of taxa with environmental sex-determination.